Programmable crystal oscillator

ABSTRACT

A programmable crystal oscillator is provided having a memory for storing frequency-defining parameters. Typically, one of these parameters is used to program an adjustable capacitive load circuit coupled to a crystal to thereby adjust the crystal source frequency. Additional parameters are used to program the output frequency of a phase locked loop circuit coupled to receive the adjusted source frequency. A further parameter can also be used to divide the output frequency of the phase locked loop circuit to supply a specified output frequency. The oscillators can be manufactured as generic programmable crystal oscillators without regard for output frequency and then quickly programmed to produce customer-specified output frequencies with a high degree of accuracy.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to programmable crystal oscillators. In particular, the present invention is directed toward a programmable crystal oscillator having an adjustable capacitive load circuit coupled to the crystal.

[0002] Crystal oscillators are widely used to generate timing signals for electronic hardware, such as computers, instrumentation, and telecommunications equipment.

[0003] Crystal oscillators typically include a quartz crystal and an oscillator circuit, which electrically excites the crystal so as to generate an oscillating signal at a resonant frequency determined by physical characteristics of the crystal. The oscillator circuit or a separate output circuit (buffer) wave-shapes the oscillating signal into a timing pulse train acceptable to the electronic hardware.

[0004] Timing frequencies are specified by the electronic hardware manufacturers and thus vary over a wide frequency range. However, a crystal's resonant frequency is determined by its physical characteristics, e.g., size, shape, crystalline structure, etc. Trimming the crystal's resonant frequency can be achieved by selective metal plating the crystal faces. Consequently, the manufacture of crystal oscillators is an involved process that is both time consuming and costly. Thus, suppliers of crystal oscillators stock large numbers of crystal oscillators manufactured to a variety of standard output frequencies. However, if a customer requires a custom frequency, a manufacturer generally must “start from scratch” by dicing an ingot into crystal wafers of specific dimensions and then subjecting the crystal wafers to numerous processing steps (lapping, etching, and plating), all designed to achieve the custom output frequency. Custom crystal oscillators come at premium prices and require long manufacturing leadtimes (months).

[0005] Since virtually all crystals are capable of oscillating, manufacturing yield is quite high. However, if the crystal's resonant frequency cannot be trimmed to meet one customer's frequency specification, the crystals are typically inventoried in the hope that they can be used to meet another customer's frequency specification. In the case of custom crystal oscillators, it is not uncommon for manufacturers to produce an oversupply of custom crystals to ensure a sufficient volume of crystals capable of meeting customer requirements in terms of both output frequency and quantity. The excess crystal oscillators are then placed in inventory. Maintaining large crystal inventories represents a significant manufacturing expense.

SUMMARY OF THE INVENTION

[0006] Systems and methods consistent with this invention resolve certain disadvantages of conventional approaches by providing a timing circuit comprising a crystal for generating a source frequency, an oscillator circuit coupled to the crystal, and a programmable load circuit coupled to the crystal. A frequency multiplier circuit is coupled to the oscillator circuit to receive the adjusted source frequency. Further, a programming circuit is included to supply first programming data to the programmable load circuit to adjust the crystal source frequency and second programming data to the frequency multiplier circuit. The frequency multiplier circuit, in turn, supplies an output frequency equal to a product of the adjusted source frequency and a multiplication factor designated by the second programming data.

[0007] Advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Additionally, advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.

[0008] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

[0009] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a plan view illustrating an exemplary package configuration for a programmable crystal oscillator package in accordance with an embodiment of the present invention;

[0011]FIG. 2 is a block diagram of a programmable crystal oscillator in accordance with an embodiment of the present invention;

[0012]FIG. 3 is a detailed circuit schematic of a programmable capacitive load circuit included in the programmable crystal oscillator of FIG. 2; and

[0013]FIG. 4 is a schematic diagram of a phase locked loop circuit included in the programmable crystal oscillator of FIG. 2.

DETAILED DESCRIPTION

[0014] Reference will now be made in detail to the presently preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

[0015] Consistent with the present invention, a programmable crystal oscillator is provided with a memory for storing output frequency-defining parameters. Typically, one of these parameters is used to program an adjustable capacitive load circuit coupled to the crystal, thereby to adjust the source (resonant) frequency of the crystal. Additional parameters are used to program the output frequency of a phase-locked loop circuit coupled to receive the adjusted source frequency. A further parameter can also be used to divide the frequency output of the phase-locked loop circuit to thereby supply the desired output frequency. As a result, by storing appropriate parameters as program data in the memory and programming in accordance with these parameters, nearly any crystal capable of oscillation may be utilized in a crystal oscillator in accordance with the present invention, which can then be programmed to generate a wide range of specified output frequencies. Moreover, such frequency programming can be performed expeditiously and at little expense.

[0016] An embodiment of a programmable crystal oscillator, in accordance with the present invention, is illustrated in FIG. 1. This oscillator 20 may be produced in a variety of industry standard sizes and in two basic package configurations, pin through and surface mounted (SMD), depending upon the manner in which the oscillator is to be mounted in its particular application. The illustrated embodiment has six input/output (I/O) terminals, consisting of a Signature clock terminal 21, a dedicated Program input terminal 22, a ground (VSS) terminal 23, a supply voltage (VDD) terminal 24, a Signature output terminal 25, and a clock signal output (F_(out))/programming clock pulse input (CLK_(in)) terminal 26. As will be described in detail below, programming data is entered via terminal 22 at a timing controlled by clock pulses (CLK_(in)) applied to terminal 26.

[0017] When programmable crystal oscillator 20 is programmed by the programming data, it produces a clock signal output (F_(out)) on terminal 26 of a programmed frequency conforming to a customer specified target frequency anywhere within a wide range, e.g., 380 KHz to 175 MHz, with an accuracy of ±100 ppm (parts per million) or better. In terms of percentage, 100 ppm is equal to ±0.01% of the target frequency. In accordance with a feature of the present invention, crystal oscillator 20 includes a programmable read only memory (PROM) 50, (FIG. 2), into which programming data in the form of customer data may be entered via Program terminal 22, under timing control imposed by clock pulses (CLK_(in)) applied to terminal 26 by the manufacturer at the time the oscillator is programmed. Thereafter, the customer data may be read out on terminal 25 by applying clock pulses to terminal 21. If this Signature data feature is omitted, the crystal oscillator package configuration illustrated in FIG. 1 may be reduced to four terminals.

[0018] Programmable crystal oscillator 20, illustrated in greater detail by the block diagram of FIG. 2, includes a crystal blank 30 electrically connected between pads 31 and 32 on an integrated circuit chip for excitation by an oscillator circuit 34 and thus to generate a source oscillating signal. This oscillator circuit includes an arrangement of resistor, capacitor, and inverter components well known in the crystal oscillator art and, thus, need not be described here. The frequency of the source oscillating signal, appearing at the output of oscillator circuit 34 as a reference frequency signal F_(ref,) is largely determined by the physical characteristics of the crystal wafer.

[0019] In accordance with a feature of the present invention, programmable crystal oscillator 20 accommodates a wide range of source frequencies, e.g.. 5.6396 MHz to 27.3010 MHz. That is, the source frequency may vary from crystal to crystal within this range without jeopardizing the ability of crystal oscillator 20 to be programmed to output clock signals at any target frequency specified by a customer within, for example, a 380 KHz-175 MHz range, with the industry standard accuracy of at least 100 ppm. In fact, the diverse crystal source frequencies need not be known in advance of programming.

[0020] Still referring to FIG. 2, oscillator circuit 34 outputs the reference frequency F_(ref), which is applied to a frequency multiplier 36, illustrated in greater detail in FIG. 4. The frequency multiplier outputs clock signals at a frequency F_(pll) to a frequency divider 38, which divides the frequency F_(pll) by a programmable parameter N, received from programming network 42, to produce clock signals F_(out) of a programmed frequency conforming to customer specification. The F_(out) and F_(ref) signals are applied as separate inputs to a multiplexer 40. Under the control of program control logic in programming network 42, imposed over line 43, multiplexor 40 outputs either clock signals F_(out) or F_(ref) through an output buffer 44 and onto terminal 26.

[0021] In accordance with another feature of the present invention, crystal oscillator 20 further includes a pair of load circuits 46 and 48 that may be programmed, if necessary, to adjust the capacitive load on crystal 30 and, in turn, pull the crystal source frequency into a range of frequencies conducive to optimal programming of crystal oscillator 20 to achieve not only the output frequency accuracy specified by a customer, but also a low phase locked loop frequency conducive to stable operation of frequency multiplier 36.

[0022] Load circuits 46 and 48 are illustrated in greater detail in FIG. 3. Load circuits 46 and 48 can include, for example, pluralities of capacitors 77 ₁ to 77 _(n) and 87 ₁ to 87 _(n), respectively. Capacitors 77 ₁ to 77 _(n) are respectively connected between ground and source electrodes of first switching elements or transistors 78 ₁ to 78 _(n), and capacitors 87 ₁ to 87 _(n) are respectively connected between ground and source electrodes of second switching elements or transistors 88 ₁ to 88 _(n). As further shown in FIG. 3, the gate of each of transistors 78 ₁ to 78 _(n) is connected by a corresponding one of output leads 76 ₁ to 76 _(n) from programming network 42, indicated collectively at 76 in FIG. 2. The drain electrodes of these transistors are connected in common to a source frequency input line 79 connecting crystal pad 31 to oscillator circuit 34. In a similar fashion, the gate electrodes are respectively connected to output leads 86 ₁ to 86 _(n) from programming network 42, (collectively indicated at 86 in FIG. 2), and the drain electrodes of transistors 88 ₁ to 88 _(n) are connected in common to source frequency input line 89 connecting crystal pad 32 to oscillator circuit 34. Additionally, fixed capacitors 75 and 85 (each having a capacitance of, e.g., 35 pf) are provided as a nominal capacitance load for crystal blank 30.

[0023] In response to a parameter stored in memory network 42, selected output lines 76 ₁ to 76 _(n) and 86 ₁ to 86 _(n) are driven high to activate or turn-on corresponding ones of transistors 78 ₁ to 78 _(n) and 88 ₁ to 88 _(n). As a result, those of capacitors 77 ₁ to 77 _(n) and 87 ₁ to 87 _(n) associated with the activated transistors are coupled to one of source frequency input lines 79 and 89. Thus, the capacitive loading of crystal blank 30 can be set in accordance with the parameter stored in memory network 42, and the crystal source frequency can therefore be “pulled” into a desired range, leaving gross output frequency adjustment to be accomplished by multiplier circuit 36. If no crystal frequency pulling is necessary, none of transistors 78 ₁ to 78 _(n) and 88 ₁ to 88 _(n) is activated.

[0024] Each of capacitors 77 ₁ to 77 _(n) and 87 ₁ to 87 _(n) may have a capacitance of, for example, 5 pf. Moreover, capacitors 77 ₁ to 77 _(n) and 87 ₁ to 87 _(n) are symmetrically connected into source frequency input lines 79 and 89, respectively, such that for each one of capacitors 77 ₁ to 77 _(n) connected to source frequency input line 79, a corresponding one of capacitors 87 ₁ to 87 _(n) is connected to source frequency input line 89. Accordingly, pairs of capacitors 77 ₁ to 77 _(n) and 87 ₁ to 87 _(n) (one from each group of capacitors 77 ₁ to 77 _(n) and 87 ₁ to 87 _(n) ) are respectively connected to input lines 79 and 89, and the incremental change of capacitance associated with each pair of capacitors may be 2.5 pf. As noted previously, this capacitance loading adjustment is effective to pull the crystal source frequency up or down as required to adjust the reference clock signal frequency F_(ref) to a value appropriate for optimal frequency programming of crystal oscillator 20.

[0025] As seen in FIG. 4, frequency multiplier 36 includes a frequency divider 52 that divides the reference frequency F_(ref) by a programmable parameter Q stored in programming network 42 and applies the resultant clock signal frequency to a phase detector 54 of a phase locked loop (PLL). The phase locked loop includes a charge pump 56, a loop filter 60, and a voltage controlled oscillator 58 that produces the clock signal frequency F_(pll) going to frequency divider 38 in FIG. 2. This clock signal frequency F_(pll) is also fed back through a frequency divider 64 to a second input of phase detector 54. Divider 64 divides the F_(pll) frequency by a programmable parameter P also stored in programming network 42.

[0026] During operation, parameters Q and P, further defined below, are supplied to program divider circuits 52 and 64, respectively, from programming network 42. Reference frequency F_(ref) is thus divided by the programmed parameter Q in divider circuit 52. As noted above, F_(ref) corresponds to the source frequency of crystal 30, and is therefore typically in a range of 5.64 MHz to 27.3 MHz. If Q is an integer within a range of 132 to 639, the quotient F_(ref)/Q, i.e., the output of divider 52, can be made to fall within the range of 32 KHz to 50 KHz, regardless of the crystal source frequency. The quotient F_(ref)/Q is the loop frequency of the phase locked loop circuit shown in FIG. 4.

[0027] The loop frequency is supplied to phase comparator 54, along with the output of divider 64, which outputs a control signal typically at a frequency substantially equal to the difference between the output frequencies of dividers 52 and 64. The control signal, in turn, is supplied to charge pump 56 which outputs a DC signal proportional to the frequency of the control signal. The loop filter 60 is provided at the output of charge pump 56 in order to define the response of the phase-locked loop circuit.

[0028] The DC signal is next supplied to voltage controlled oscillator (VCO) 58, which outputs clock signal F_(pll) of a frequency controlled by the potential of the DC signal. Clock signal F_(PLL) is next fed back to one of the inputs of phase detector 64 via P divider 64. Accordingly, with this phase locked loop arrangement, F_(PLL) is equal to the product of the loop frequency multiplied by programmable P, an integer in the range of 2048 to 4097, so that F_(PLL) is within the range of 87.5 MHz to 175 MHz. Clock signal F_(PLL) is also supplied to divider circuit 38 (see FIG. 2), where it is divided by any programmed one of the following integer parameters retrieved from programing network 42: 1, 2, 4, 8, 16, 32, 64, 128 or 256, and then outputted as frequency F_(out) through multiplexor 40 and output buffer 44.

[0029] Phase detector 54, charge pump 56, loop filter 60 and VCO 58 are intended to represent a broad category of circuits utilized to phase lock two input signals, which are well known in the art and will not be described further.

[0030] Generally, the output frequency F_(PLL) and F_(ref) satisfy the following equation:

F _(PLL) =F _(ref)(P/Q).

[0031] Thus, F_(PLL) is a multiple of the loop frequency. Accordingly, for relatively low loop frequencies, F_(PLL) can be finely adjusted in small increments of the loop frequency. If the loop frequency is too low, the phase locked loop can become unstable, resulting in jitter. Thus, it has been determined that an optimal loop frequency range is 32 KHz to 50 KHz, with a preferred range of 42.395 KHz to 43.059 KHz. Loop frequencies above this range, but less than 200 KHz, can also be used with output frequency resolution better than that obtainable by conventional crystal oscillator PLLs.

[0032] Conventional crystal oscillator phase locked loops used in digital circuit timing applications, however, operate at a significantly higher loop frequency, i.e., greater than 200 KHz. At these frequencies, such conventional crystal oscillators cannot achieve the same high frequency resolution attainable with the present invention. Typically, polysilicon capacitors, for example, and other low noise components, are incorporated into the phase-locked loop typically used in the present invention so that it can operate with a loop frequency within the preferred range of 42.395 KHz to 43.059 KHz with relatively little jitter.

[0033] As discussed in greater detail in applicants' related patent application, entitled WORLDWIDE MARKETING LOGISTICS NETWORK INCLUDING STRATEGICALLY LOCATED CENTERS FOR FREQUENCY PROGRAMMING CRYSTAL OSCILLATORS TO CUSTOMER SPECIFICATION, Serial No. ______, filed concurrently herewith, the disclosure thereof being incorporated herein by reference, the P, Q, and N parameters alone may be insufficient to achieve an output frequency sufficiently close, e.g., within 100 parts per million (100 ppm) of a specified target frequency. In which case, the crystal source frequency is pulled, as discussed above, to bring the resulting output frequency into the acceptable accuracy range.

[0034] Thus, the programmable crystal oscillator in accordance with the present invention can be used to generate an output frequency based on a wide range of crystal source frequencies. The output frequency is obtained by adjusting the source frequency with a programmable capacitive load, and operating the phase-locked loop circuit at a relatively low loop frequency. As a result, for any crystal having a source frequency within the relatively wide range of, e.g., 5.6396 MHz to 27.3010 MHz, crystal oscillator output frequencies within 100 ppm or less of a specified target frequency can be achieved by simply storing appropriate P, Q, N and crystal pulling parameters in a PROM included in programming network 42. As pointed out in the cited copending application, crystal oscillators 20 may be manufactured as generic programmable crystal oscillators, without regard to customer-specified output frequencies and simply programmed in a matter of seconds to generate output frequencies in accordance with customer specifications. Consequently, there is no need to manufacture the crystals to a plurality of standard frequencies, thus simplifying, expediting, and cost-reducing the manufacturing process. Leadtimes from customer purchase order to product delivery may thus be dramatically reduced.

[0035] While the present invention has been described in the context of using a standard microprocessor-type crystal blank that oscillates in the range of 5.6396 MHz to 27.3010 MHz, as noted above, it will be understood that the present invention may be achieved using an industry standard watch crystal mass produced to oscillate at 32.768 KHz. In this case, the desired low phase-locked loop frequency may be achieved without the need for frequency divider 52 in frequency multiplier 36 of FIG. 4. Crystal blank 30 would then, in effect, be coupled in direct drive relation with the phase-locked loop circuit. Since watch crystals, by virtue of mass production, are significantly less expensive than microprocessor-type crystals, further economies in the production of programmable crystal oscillators in accordance with the present invention may be achieved.

[0036] It will be apparent to those skilled in the art that various modifications and variations can be made in the programmable crystal oscillator of the present invention and in construction of this programmable crystal oscillator without departing from the scope or spirit of the invention.

[0037] Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 

What is claimed is:
 1. A timing circuit comprising: a crystal for generating a source frequency; a programmable load circuit coupled to said crystal; an oscillator circuit coupled to receive said source frequency; a frequency multiplier circuit coupled to said oscillator circuit for receiving said source frequency; and a programming circuit configured to supply first programming data to said programmable load circuit to adjust said source frequency and second programming data to said frequency multiplier circuit, such that said frequency multiplier circuit supplies an output frequency which is substantially equal to a product of said adjusted source frequency and a multiplication factor designated by said second programming data.
 2. A timing circuit in accordance with claim 1 , wherein said programmable load circuit is a programmable capacitive load circuit.
 3. A timing circuit in accordance with claim 2 , wherein said source frequency is supplied over a source frequency line coupled to said crystal, said programmable capacitive load circuit comprises: a plurality of capacitors, selected ones of said plurality of capacitors being coupled to said source frequency line.
 4. A timing circuit in accordance with claim 3 , wherein said programmable capacitive load circuit further comprises: a plurality of switching elements, each of which having a first terminal connected in common to said source frequency line, and a second terminal coupled to a corresponding one of said plurality of capacitors.
 5. A timing circuit in accordance with claim 4 , wherein said programming circuit generates a plurality of output signals in accordance with said first programming data stored therein, said plurality of output signals being supplied to said programmable capacitive loading circuit to activate selected ones of said plurality of switching elements, such as to connect corresponding ones of said capacitors to said source frequency line.
 6. A timing circuit in accordance with claim 4 , wherein each of said plurality of switching element comprises an MOS transistor.
 7. A timing circuit in accordance with claim 1 , wherein said frequency multiplier circuit includes a phase locked loop circuit.
 8. A timing circuit in accordance with claim 1 , wherein said second programming data comprises a first parameter P and a second parameter Q, said output frequency (F_(out)) and said adjusted source frequency (F_(adj)) satisfying: F _(out) =F _(adj)(P/Q).
 9. A timing circuit in accordance with claim 7 , wherein said frequency multiplier loop circuit comprises: a first divider for dividing said adjusted source frequency by a first parameter of said second programming data to generate a first frequency; a second divider included in said phase locked loop circuit for dividing said output frequency by a second parameter of said second programming data to generate a second frequency; and a detector included in said phase locked loop circuit and coupled to receive said first and second frequencies, said detector outputting a control signal in response to said first and second frequencies for controlling a generation of said output frequency.
 10. A timing circuit in accordance with claim 9 , further comprising: a third divider circuit, coupled to an output of said phase locked loop circuit and said programming circuit, for dividing said output frequency by a third parameter of said second programming data stored in said programming circuit.
 11. A timing circuit in accordance with claim 7 , wherein said adjusted source frequency is supplied to a divider circuit to generate a loop frequency of said phase locked loop circuit less than 200 KHz.
 12. A timing circuit in accordance with claim 11 , wherein said loop frequency is within a range of 32 KHz to 50 KHz.
 13. A timing circuit in accordance with claim 11 , wherein said loop frequency is within a range of 42.395 KHz to 43.059 KHz.
 14. A timing circuit in accordance with claim 9 , wherein said detector outputs said control signal in accordance with a phase difference between said first and second frequencies, said phase locked loop circuit further comprising: a charge pump circuit receiving said control signal and outputting a DC signal in response thereto; a loop filter; and a voltage controlled oscillator coupled to said charge pump through said loop filter, said voltage controlled oscillator generating said output frequency under a control of said DC signal.
 15. A timing circuit in accordance with claim 1 , further comprising a dedicated external programming terminal for entering said first and second programming data for storage in a programmable read only memory included in said programming circuit.
 16. A programmable timing circuit comprising: a crystal; an oscillator circuit coupled to excite said crystal into generating a source frequency; a capacitive load circuit coupled to said crystal and configured to be programmable to a desired load capacitance, such as to selectively adjust said source frequency; and a phase locked loop circuit coupled said oscillator circuit, said phase locked loop circuit generating an output frequency as a product of said adjusted source frequency and a multiplication factor.
 17. A programmable timing circuit in accordance with claim 16 , further comprising: a memory, coupled to said phase locked loop circuit and to said programmable capacitive load circuit, for storing programming data including a first parameter for programming said capacitive load circuit to said desired load capacitance and a second parameter for programming said phase locked loop circuit to said multiplication factor.
 18. A programmable timing circuit in accordance with claim 17 , wherein said memory comprises a PROM.
 19. A programmable timing circuit in accordance with claim 17 , wherein said crystal is coupled to said oscillator circuit by an input line, said programmable capacitive load circuit comprising: a plurality of capacitors; and a plurality of switching elements, each having a first terminal coupled to a corresponding one of said plurality of capacitors and a second terminal coupled to said input line, said memory supplying said first parameter as a plurality of signals to activate selected ones of said plurality of switching elements, thereby coupling said corresponding ones of said plurality of capacitors to said input line.
 20. A programmable timing circuit in accordance with claim 19 , wherein each of said plurality of switching elements comprises an MOS transistor.
 21. A programmable timing circuit in accordance with claim 17 , further comprising a dedicated programming terminal accommodating external entry of said programming data for storage in said memory.
 22. A programmable crystal oscillator, comprising: a crystal having an associated source frequency; an oscillator circuit; input lines coupling said crystal to said oscillator circuit; a programmable capacitive load, coupled to said input lines, for adjusting said source frequency according to a loading parameter; a phase locked loop circuit coupled to receive said adjusted source frequency and to generate a loop frequency in response thereto, said phase locked loop circuit multiplying said loop frequency by a frequency parameter to generate a desired output a frequency; and a programming circuit coupled to said programmable capacitive load circuit and to said phase locked loop circuit, said programming circuit storing said loading and frequency parameters for respectively programming said capacitive load circuit and said phase locked loop circuit.
 23. The programmable crystal oscillator in accordance with claim 22 , wherein said crystal is an industry standard watch crystal having an associated source frequency of essentially 32.768 KHz.
 24. A timing circuit in accordance with claim 1 , further comprising: a first PROM for storing identification data; a program terminal for programming said first PROM with said identification data; and a first output terminal for reading said identification data out of said first PROM.
 25. A timing circuit in accordance with claim 24 , further comprising: a second PROM programmed by said first and second programming data entered through said program terminal; and a second output terminal for outputting said output frequency and for receiving clock pulses to clock said identification data into first PROM and to clock said first and second programming data into said second PROM.
 26. A timing circuit in accordance with claim 25 , further including a second input terminal for receiving clock signals to read said identification data from said first PROM out onto said first output terminal. 